There are several types of fans, designed in different ways based on different areas of application, fluid, flow rate and pressure increase capacity. Centrifugal fans are one of several types of fans, which amongst other things are especially suitable for use as lift fans for hovercraft.
Larger centrifugal fans are generally constructed of metallic materials, such as steel, acid-proof steel or aluminium. These fans typically have a very high rotation speed that generates enormous centrifugal forces, corresponding to 1000-4000 times the gravitational acceleration on the fan blades. The fan construction must therefore be extremely robust to enable those great forces to be absorbed in the structure.
U.S. Pat. No. 3,171,586 describes a metallic centrifugal fan with a rotor and airfoil blades bolted thereto. The blades are fixed to a side plate and to a back plate by means of studs and nuts. These bolts extend on part of the blade and are intended to provide hard joints, where short studs are subjected to shear and tension with no bending. Said studs do not provide any strengthening of the blades.
For use in military vessels there are also requirements regarding magnetic signature from the vessel and its systems. Rotating metal structures, for example, produce a disturbance in the earth's magnetic field that can be intercepted by special sensors. It is therefore vital that fans produce minimal electrical eddy currents. This is solved by using materials with low electrical conductivity in the construction of the fan, where in addition each individual part is electrically insulated from the others.
Existing designs of centrifugal fans in composite have employed an unsatisfactory method for attaching the blades to side disc and centre wheel, whereby substantial shear forces are generated in the bolt connection. This has resulted in a short service life and also several cases of fan breakdown.
U.S. Pat. No. 6,402,467 describes an impeller including vanes, first and second coaxial discs formed from composite material on opposite sides of the vanes to fix the vanes in position and a reinforcing collar. The inner surfaces of the discs comprise depressions intended to receive wings of vanes, these are vacuum bonded to the discs. If one vane is defect and must be replaced, the discs must be removed.
The object of the present invention is thus to provide a robust centrifugal fan which is entirely or partly made of composite materials and which avoids the above-mentioned disadvantages related to the attachment/removal of the blades and at the same time provides correct load transfer from the blades. It is a further object to obtain a centrifugal fan with a low magnetic signature.
Furthermore, the centrifugal fan according to the invention has a low moment of inertia, thus enabling start-up of the fan to be implemented without a complicated coupling and/or gear device between fan and drive motor.